If the barrel length is 18" or longer, it may be called "a home protection & insurance device". If the barrel length is less than 18" it is called "illegal" by the atf boys in washington. A more polite term would be to call any type of shotgun a "scatter-gun" when liberals are nearby. I know this for a fact from having a table at local gunshows many years ago during the 1980's & 1990's.

For example: "sonic sound suppressor" is more socially acceptable than "silencer"
"home defense scattergun" is more acceptable than "sawed-off"

i know this junk is really an aggravation to true gun-folks, but i have been very successful in turning around many liberals by simply educating them to the truth.

Note also that a true carbine is not just a short rifle, but rather a rifle of reduced power.

Doesn't shortening the barrel usually reduce the muzzle velocity? Or do you mean that a carbine is specifically a short barrelled, lower power version of a given rifle (as opposed to a gun that was originally designed with a short barrel)?

Yes indeed that used to be the case, but ask some of the resident handloaders here and they can explain how powder charge and bullet weight can make a shorter barrel shoot to within a few hundred fps of the longer barrel with a "standard" load. Its majic I think.
Eli